Physics
One well-accepted approach to attracting an asteroid towards a spacecraft is the gravity tractor. Essentially, a spacecraft approaches a small asteroid, and then the spacecraft's own gravity pulls the asteroid. The effect is tiny, but over a very long time, provided the asteroid isn't too big, the spacecraft can change the direction of the asteroid by a very small degree. This has been proposed as a way to prevent an asteroid from colliding with the Earth where a several decades long warning is raised as to a future collision.
A force field confined to a collimated beam with clean borders, is one of the principal characteristics of tractor and repulsor beams. Several theories that have predicted repulsive effects, do not fall within the category of tractor and repulsor beams because of the absence of field collimation. For example, Robert L. Forward, Hughes Research Laboratories, Malibu, California, showed general relativity theory allowed the generation of a very brief impulse of a gravity-like repulsive force along the axis of a helical torus containing accelerated condensed matter. The mainstream scientific community has accepted Forward’s work. A variant of Burkhard Heim’s theory by Walter Dröscher, Institut für Grenzgebiete der Wissenschaft (IGW), Innsbruck, Austria, and Jocham Häuser, University of Applied Sciences and CLE GmbH, Salzgitter, Germany, predicted a repulsive force field of gravitophotons could be produced by a ring rotating above a very strong magnetic field. Heim’s theory, and its variants, have been treated by the mainstream scientific community as fringe physics. But, the works by Forward, Dröscher, and Häuser could not be considered as a form of repulsor or tractor beam because the predicted impulses and field effects were not confined to a well defined, collimated region.
The following are a summary of experiments and theories that resemble repulsor and tractor beam concepts:
Read more about this topic: Tractor Beam
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